ci/cd: sign commits from goreleaser

**Motivation:** support confidence.

**Research:**

- https://httgp.com/signing-commits-in-github-actions/
- https://github.yungao-tech.community/t/sign-bot-commit-in-actions/17896

**Related issues:**

- kamilsk/homebrew-tap/issues/8
- octolab/homebrew-tap/issues/3